That’s what I wanted to do in the first place, but decided to ask here first
Meanwhile I am doing some other migration-related things, but yes, in the end it likely will be a mass sed for replacing my old custom summary divider.
Update. Here’s the sed command I used on Mac OS (ran in the same folder where my post files were):
find . -type f -iname "*.md" -exec sed -i '' -e 's/<!-- some-other-value -->/<!--more-->/' {} \;